The End of PEDOT:PSS in Perovskite Tandems

The pursuit of high-efficiency solar energy is hitting a structural bottleneck. While the industry has relied on PEDOT:PSS for its transparency and conductivity, its acidic and hygroscopic nature actively degrades sensitive perovskite layers scientists have now developed a solution that removes this degradation risk while maintaining high performance.

Researchers from the Hong Kong University of Science and Technology (HKUST) have engineered a PEDOT:PSS-free all-perovskite tandem solar cell achieving 29.1% efficiency. The team replaced the traditional conductive polymer blend with a phenothiazine-functionalized phosphonic acid monolayer.

The problem with PEDOT:PSS is not just its chemistry, but its impact on the device's lifecycle. In tandem architectures, this material can absorb moisture and interact unfavorably with perovskite precursors. These interactions promote phase segregation during crystallization, which undermines both performance and stability. For a technology aiming to surpass the efficiency limits of single-junction cells, such interfacial instability is a non-starter.

The HKUST team addressed this by using 4PAPT, a phenothiazine-functionalized self-assembled monolayer. This substitution does more than just remove an acidic component; it promotes a direct phase transition, improves crystal orientation, and suppresses non-radiative recombination losses. By utilizing in-situ characterization, the researchers demonstrated that removing PEDOT:PSS prevents the unstable crystallization pathway typically seen in mixed tin-lead perovskite films.

This development shifts the focus from merely stacking absorbers to managing the buried interfaces of the narrow-bandgap tin-lead perovskite subcell. If these all-perovskite tandems can maintain their 29.1% efficiency while utilizing low-cost manufacturing, the path toward stable, lightweight, and high-efficiency solar deployment becomes much clearer.

The question for the industry remains: can this monolayer approach scale to the same level of reproducibility as the solution-processed films that made PEDOT:PSS the standard?